1. Field of the Invention
The present invention relates to switched power supplies, and more specifically, to a circuit for a flyback converter which incorporates a switched shunt inductance. The shunt inductance is used to produce a zero volt switching condition across the power switch of the power supply. This permits the power switch to be resonantly switched on with a zero voltage condition, thereby reducing the power loss associated with changing the state of the switch. The circuit may be used in conjunction with an active clamp which provides shaping of the output voltage and a large value for the resonant snubber capacitor or with a flyback converter circuit which does not use clamp elements. The switched shunt inductance reduces the power losses associated with changing the state of the primary switch compared to conventional zero volt switching flyback converter circuits, and improves the overall efficiency of the converter.
2. Description of the Prior Art
Switching or "switch mode" power supplies use a semiconductor device as a power switch to control the application of a voltage to a load. A "flyback" or "buck-boost" converter is used to produce an output (or load) voltage which is of the opposite polarity and which may be of higher, lower, or equal voltage than the input voltage supplied by the input power supply. FIG. 1 is a schematic diagram showing a basic circuit for a prior art flyback converter 100. The operation of power switch Q1 102 is controlled by applying a control signal waveform to control node 103 (e.g., the gate of a FET device). Note that although switch 102 is depicted in the figure as a FET device, it may take other forms such as a bipolar junction transistor, etc. Note also, that although not shown in the figure, a FET device has an associated body diode and stray capacitance.
When switch Q1 is turned "on", i.e., conducting, the input voltage supplied to the input nodes (labeled "SUPPLY" in the figure) is applied across the primary winding ("P1") of power transformer 104. Under steady-state conditions, the current (I.sub.p) in P1 will increase linearly with time to a peak value as energy from the input supply is stored in its magnetic core. This is described by the relationship V=LdI.sub.p /dt, where L is the value of the magnetizing inductance of the primary winding.
In the situation described, rectifier D1 106 will be reversed biased and thus not conducting. Therefore, current will not flow in the secondary winding ("S1") of power transformer 104 when power switch 102 is on and current is flowing in the primary winding. In addition, under steady-state conditions, current will flow from output capacitor C.sub.1 108 to the load ("LOAD") attached across the output nodes of flyback converter 100. This causes output capacitor 108 to discharge.
When power switch Q1 102 is turned "off", i.e., not conducting, current is no longer supplied by the power supply to the primary winding. The reduction in current flowing in the primary winding causes transformer 104 to produce a back emf which produces an increase in the current (I.sub.S) flowing through secondary winding S1. This causes a reversal of the voltages on the windings. When the induced flyback voltage reaches a level greater than the sum of the output voltage across output capacitor 108 and the diode drop across D1, it causes rectifier D1 106 to become forward biased, permitting the primary current (and hence the energy stored in the magnetizing inductance of transformer 104) to be transferred to the secondary winding and ultimately to output capacitor 108 and the load (which is represented as a resistance in the figure). Power switch Q1 102 is then turned back on by application of a suitable signal to control node 103 to start another cycle of the converter.
The flyback converter output voltage, V.sub.out, is determined by the duty ratio of power switch Q1 and the input supply voltage, SUPPLY, according t o EQU V.sub.out =SUPPLY*D,
where D is the duty cycle of the switch and is defined as t.sub.on /(t.sub.off), with t.sub.on being the "on" time of the switch during a cycle and t.sub.off being the "off" time during a cycle. Turns ratio of the transformer is 1:1 and voltage output and "V.sub.OUT " in text are the same magnitude.
There are two types of flyback converters, with the two differing in their mode of operation. A discontinuous flyback converter operates in a mode in which all the energy stored in the transformer during an energy storage period (the "on" period of the power switch) is transferred to the output during the flyback (the "off" period of the power switch) period. A continuous flyback converter operates in a mode in which part of the energy stored in the transformer during an energy storage period (the "on" period of the power switch) remains in the transformer at the beginning of the next "on" period. Thus, in a continuous mode flyback converter, the next cycle begins before the current in the secondary winding of the power transformer falls to zero.
When power switch Q1 is turned on, energy is stored in both the transformer's core (the magnetizing inductance) and in the primary side leakage inductance of the transformer (not shown) which is connected in series with the magnetizing inductance. When the power switch is turned off, the energy in the core (magnetizing inductance) is coupled to the secondary winding and output circuit. However, the leakage inductance and stray capacitance of the power switch form a high-frequency LC resonant circuit, so that the energy stored in the leakage inductance "rings" with the stray capacitance of the power switch (i.e., the stored energy causes a voltage waveform which oscillates with a frequency dependent upon the value of the leakage inductance and stray capacitance). This causes the voltage across the switch to increase and results in greater power loss and possible damage to the switch. The "ringing" voltage is typically dissipated by a "snubber" circuit (e.g., a capacitor) connected to the power switch, which acts to damp the resonant circuit. Although use of a snubber circuit can reduce damage to the power switch, it still results in dissipated energy and reduces the overall efficiency of the converter. This is because of the losses associated with the components of the snubber circuit, e.g., the I.sup.2 R loss of the resistive element and the energy stored in the capacitive element which is not fully discharged prior to the start of a new converter cycle.
A drawback of switch mode power circuits as described above is that the switching devices in such switch mode power converters are subjected to high stresses and potentially high power loss as a result of the switch being changed from one state to another while having a significant voltage across it. These effects increase linearly with the switching frequency of the waveform used to control the power switch. Another drawback of switched power circuits is the electromagnetic interference (EMI noise) arising from the high values of dI/dt and dV/dt caused when the switch changes state. This interference may cause a disruption in the operation of other, nearby circuits or devices.
The noted disadvantages of switch mode power converters can be reduced if each power switch in the circuit is caused to change its state (from "on" to "off" or vice versa) when the voltage and/or current through it is zero or at a minimum. Such a control scheme is termed "zero-voltage" (ZVS) and/or "zero-current" (ZCS) switching. In the case of switching at a minimum voltage, the control scheme is termed "low-voltage" switching. It is thus desirable to switch the power switching device(s) at instances of zero or minimum voltage in order to reduce stress on the switch(es) and power loss of the converter. This will help to increase the efficiency of the converter.
One method of implementing zero voltage switching is to provide a voltage signal across the power switch which passes through a zero value. This can be achieved in the case of discontinuous mode flyback converters by placing a passive voltage clamp circuit in parallel with the primary winding of the transformer to create a resonant circuit. Energy stored in the transformer's leakage inductance is transferred to the clamp circuit capacitor and back to the primary winding during the flyback cycle, causing a fluctuation in the voltage applied across the power switch.
In the case of a continuous mode flyback converter, an "active clamp" circuit can be used to return the energy stored in the leakage inductance to the input supply line and to provide zero voltage switching of the power switch. Such a circuit configuration is described in U.S. Pat. No. 5,570,278, assigned to the assignee of the present invention, and the contents of which is incorporated herein by reference. FIG. 2 shows the flyback converter of FIG. 1, to which has been added an active clamp circuit designed to provide a zero volt switching signal across the power switch. The active clamp is formed from a series combination of clamp switch (S.sub.CL) and clamp capacitor (C.sub.CL) coupled in parallel across the primary winding (P1) of transformer 104. Depending upon the component values and the timing between the control signals applied to the power switch and clamp switch, in the circuit of FIG. 2, the active clamp components can serve the following functions: (1) to provide a ZVS signal across the power switch; (2) to route the energy stored in the leakage inductance to the input instead of having it be dissipated in a snubber circuit; (3) to provide shaping of the output signal; and (4) to provide a ZVS signal through the secondary side rectifier.
The clamp switch and power switch are operated in a manner which causes energy stored in the power transformer's leakage inductance to be transferred to the clamp capacitor and back to the primary winding during the flyback cycle (when switch Q1 102 is off). This will produce a reversing voltage which is applied to the series combination of the leakage inductance and primary winding of the power transformer, causing a fluctuation in the voltage across power switch Q1 102. In order to achieve ZVS behavior, active clamp switch S.sub.CL operates substantially in anti-phase relation to power switch Q1 102. The operation of switches Q1 and S.sub.CL is non-overlapping, with clamp switch S.sub.CL closing after power switch Q1 opens and opening before power switch Q1 next closes.
When active clamp switch S.sub.CL is closed (i.e., "on"), clamp capacitor C.sub.CL applies a reversing voltage to the series combination of the primary winding P1 and the leakage inductance (not shown). The reversing voltage has a polarity which is opposite to that of the voltage applied to the series combination during the preceding ON period of the power switch. By turning active clamp switch S.sub.CL off for a period of time before power switch Q1 is turned on for the next cycle, the voltage across the power switch may be reduced to zero prior to the power switch being turned on. Thus, under appropriate timing conditions between the active clamp switch and power switch, the active clamp components form a resonant circuit which provides a zero voltage signal across the power switch.
At the end of the clamp cycle, when the load has been serviced, the current reflected back to the primary circuit is a function of the "tuning" of the secondary load circuit. The clamp turn off current can be increased by changing the value of the clamp capacitor in a manner independent of the value of the load current. This ensures that a ZVS signal can be generated at higher switching frequencies.
While the addition of an active clamp circuit to a flyback converter permits continuous mode operation with ZVS of the power switch, it does not remove all of the inefficient aspects of the flyback topology. In particular, the reverse recovery losses associated with the secondary stage rectifier D1 (element 106 in FIGS. 1 and 2) can still be significant. In addition, under typical operating conditions, the current through the clamp switch can be high enough to make the I.sup.2 R losses in the equivalent series resistance of the clamp capacitor and clamp switch significant. Since the current level though the clamp switch is a function of the primary side leakage inductance and a large leakage inductance is required to generate the ZVS signal in the circuit shown in FIG. 2, this is an inherent disadvantage of using the active clamp components to generate a ZVS signal for the power switch.
What is desired are circuits for continuous flyback converters, both which use active clamp circuits and which do not use active clamp circuits, which have lower power losses and are more efficient than currently available designs of such converters.